bpo-35900: Enable custom reduction callback registration in _pickle

[pierreglaser]

The C implementation of the Pickler class does not allow to register custom pickling behavior for classes and functions.
This PR enables it by adding a hook that will call a user-defined callable for those types. If no callable is registered, then the pickler simply fallbacks the save_global method, as before.

See also:
cloudpipe/cloudpickle#253

https://bugs.python.org/issue35900

[ogrisel]

Here is a batch of comments:

[pitrou]

Here is a quick review. Summary:

• the same hook needs to be available (and tested) on the pure Python implementation as well
• the hook should be able to be specified as a method in a subclass, as well as an independent callable (therefore its signature should be just (obj), IMHO)
• some docs need to be added

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[pitrou]

@ncoghlan What do you think about the name "reducer_override" here?

[ncoghlan]

@pitrou The proposed method name seems fine to me, but I'd expect to see a docs update to https://docs.python.org/3/library/pickle.html#pickle.Pickler to mention the subclassing API.

[ogrisel]

Some more comments:

[ogrisel]

LGTM.

[pierreglaser]

I have made the requested changes; please review again
OK @pitrou @ncoghlan I think I addressed everything?

[bedevere-bot]

Thanks for making the requested changes!

@pitrou: please review the changes made to this pull request.

[pitrou]

Rebased.